What is her secret? Abbie boudreau has the story. Reporter: All eyes are on britney spears' body of work and her new music video with will.I.Am. ♪ I want to scream and shout and let it all out ♪ Reporter: She's toned. Performing her tune "scream and shout," which premiered this week on "the x factor." How does the 30-year-old mom stay in shape? It helps that britney is a fitness fanatic. Once known for 1,000 crunches a way. Britney works out every day. 30 minutes at least of cardio, And she says she feels on top of the world after she's done it. ♪ All eyes on us ♪ Reporter: It could be a rush, not unlike caramel chocolate, which britney admitted is one of her guilty pleasures. She's eliminated sugar from her diet. Britney and her fiance, jason, they workout together. They go on walks together. Britney has two sons. She's very busy with that. She has a lot on her plate. But she's most definitely dedicated to her fitness. Reporter: Last year, she told "stylist" magazine, working out can be a challenge. But a worthwhile one. Sometimes it's hard to be motiva motivated, she said. But once you do it, you feel like you can conquer the world. I think britney looks incredible in this video. She is obviously on top of her fitness game and very dedicated to it because she looks just amazing. Reporter: But there are always temptations. Can britney resist birthday cake this sunday, when she turns 31? For "good morning america," abbie boudreau, abc news, los angeles.

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